1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus, wherein a radiation image having been stored on a stimulable phosphor sheet is read out with a line sensor. This invention particularly relates to a radiation image read-out method and apparatus, wherein a correction is made for variations in sensitivity among photoelectric conversion devices of a line sensor.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, which comprises a substrate and a layer of the stimulable phosphor overlaid on the substrate. Stimulating rays, such as a laser beam, are deflected and caused to scan pixels in the radiation image, which has been stored on the stimulable phosphor sheet, one after another. The stimulating rays cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted successively from the pixels in the radiation image having been stored on the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal by photoelectric read-out means. The stimulable phosphor sheet, from which the image signal has been detected, is then exposed to erasing light, and radiation energy remaining thereon is thereby released.
Also, a novel radiation image recording and reproducing system aiming at enhancement of a detection quantum efficiency in the formation of the radiation image, i.e., a radiation absorptivity, a light emission efficiency, an emitted light pickup efficiency, and the like, has been proposed in, for example, patent literature 1. With the proposed radiation image recording and reproducing system, the radiation absorbing functions and the energy storing functions of the conventional stimulable phosphor are separated from each other, and a phosphor having good radiation absorbing characteristics and a phosphor having good light emission response characteristics are utilized respectively for radiation absorption and radiation image storage. The phosphor having good radiation absorbing characteristics (i.e., the phosphor for radiation absorption) is caused to absorb the radiation and to emit light having wavelengths falling within an ultraviolet to visible region. Also, the phosphor having good light emission response characteristics (i.e., the phosphor for energy storage) is caused to absorb the light, which has been emitted by the phosphor having good radiation absorbing characteristics, and to store energy of the emitted light. The phosphor having good light emission response characteristics, on which the energy of the emitted light has been stored, is then exposed to light having wavelengths falling within a visible to infrared region, which light causes the phosphor having good light emission response characteristics to emit light in accordance with the stored energy. The light having thus been emitted by the phosphor having good light emission response characteristics is successively detected with photoelectric read-out means, and an image signal is thereby obtained.
The image signal, which has been obtained from the radiation image recording and reproducing systems described above, is then subjected to image processing, such as gradation processing and processing in the frequency domain, such that a visible radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, can be obtained. The image signal having been obtained from the image processing is utilized for reproducing a visible image for diagnosis, or the like, on film or on a high resolution cathode ray tube (CRT) display device. In cases where the stimulable phosphor sheet, from which the image signal has been detected, is then exposed to the erasing light, and energy remaining on the stimulable phosphor sheet is thereby released, the erased stimulable phosphor sheet is capable of being used again for the recording of a radiation image.
Novel radiation image read-out apparatuses for use in the radiation image recording and reproducing systems described above have been proposed in, for example, patent literatures 2, 3, and 4. In the proposed radiation image read-out apparatuses, from the point of view of keeping the emitted light detection time short, reducing the size of the apparatus, and keeping the cost low, a line light source for irradiating linear stimulating rays onto a stimulable phosphor sheet is utilized as a stimulating ray source, and a line sensor comprising a plurality of photoelectric conversion devices arrayed along the length direction of a linear area of the stimulable phosphor sheet, onto which linear area the stimulating rays are irradiated by the line light source, is utilized as photoelectric read-out means. (The length direction of the linear area of the stimulable phosphor sheet will hereinbelow be referred to as the main scanning direction.) Also, the proposed radiation image read-out apparatuses comprise scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet. (The direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet, will hereinbelow be referred to as the sub-scanning direction.)
However, the line sensor utilized in the radiation image recording and reproducing systems described above comprises the plurality of the photoelectric conversion devices, which are arrayed in the main scanning direction, and variations in light receiving sensitivity occur among the photoelectric conversion devices. Therefore, the problems occur in that an artifact due to the variations in sensitivity among the photoelectric conversion devices is mixed in an output signal obtained from the line sensor, and an image having good image quality cannot be obtained. For example, in cases where the line sensor is constituted of a charge coupled device (CCD) image sensor, photodiodes (hereinbelow referred to as PD's) of the CCD image sensor, which PD's act as the photoelectric conversion devices, are independent of one another, and the sensitivities of the PD's are not identical with one another. Therefore, in order for an image having good image quality to be obtained, it is necessary that the outputs obtained from the PD's are corrected in accordance with the variations in sensitivity among the PD's of the CCD image sensor.
By way of example, a technique for making a correction for the variations in sensitivity among the photoelectric conversion devices of the line sensor has been proposed by the applicant in patent literature 5. With the proposed technique, light coming from a reference light source is received, and output signal components obtained from the photoelectric conversion devices are normalized with a mean value. In this manner, the correction is made for variations in sensitivity among the pixel regions of the line sensor.                Patent literature 1: U.S. Pat. Laid-Open No. 20010022349        Patent literature 2: U.S. Pat. No. 4,922,103        Patent literature 3: U.S. Pat. No. 4,816,679        Patent literature 4: Japanese Unexamined Patent Publication No. 1(1989)-101540        Patent literature 5: U.S. Patent Laid-Open No. 20020003218        
However, with the aforesaid technique for making a correction for the variations in sensitivity among the photoelectric conversion devices of the line sensor, wherein the output signal components obtained from the photoelectric conversion devices are merely normalized with the mean value, the problems occur in that a profile of the line-like reference light source remains as a correction residue, and the correction for the sensitivity cannot be performed sufficiently.
Also, with the aforesaid technique for making a correction for the variations in sensitivity among the photoelectric conversion devices of the line sensor, wherein the output signal components obtained from the photoelectric conversion devices are merely normalized with the mean value, the problems occur in that a correction cannot be made for a term due to deterioration of the reference light source with the passage of time. Therefore, in order for the correction to be made accurately, it is necessary to perform operations, wherein X-rays are uniformly irradiated to a stimulable phosphor sheet, and correction values are calculated from comparison with the signal obtained from the stimulable phosphor sheet having been uniformly exposed to the X-rays. However, the correction processing accompanying the uniform irradiation of the X-rays gives a stress to the user.